In 1866, Marx wrote to Frederick Engels, that Origin of the Species contained the basis in natural history for their political and economic system for an atheist world. Engels, the co-founder of world communism with Marx and Lenin, wrote to Karl Marx in 1859: Darwin, whom I am just now reading, is splendid (C. Zirkle, Evolution, Marxian Biology, and the Social Scene, 1959, p. 85). In 1861, Marx wrote to Engels: Darwins book is very important and serves me as a basis in natural selection for the class struggle in history (op. cit., p. 86). At Marxs funeral, Engles said that, as Darwin had discovered the law of organic evolution in natural history, so Marx had discovered the law of evolution in human history (Otto Ruhle, Karl Marx, 1948, p. 366).
Note: The above was to show why many associate Darwin with Marx. In Darwins defense though, I do not believe he intended to establish communism with his theory.